It is known now, from patents originating with our group, that catalysts the starting components of which are (a) a hydride or organometallic compound of a metal belonging to Groups I to III of the Mendelyeev Periodic System, typically an alkyl Al compound, and (b) the product obtained by contacting a compound of a transition metal, typically TiCl.sub.4 or TiCl.sub.3, with a support or carrier which is MgCl.sub.2 activated by special techniques, are used for the polymerization of olefins, e.g., ethylene, propylene or mixtures of propylene with minor amounts of ethylene, the yield of polymer obtained, in grams/gram of Ti used is much higher than the yield obtained using the catalysts without a support or with a conventional support different from the activated MgCl.sub.2.
The known crystalline modification of MgCl.sub.2 in the activated state is the .alpha.-form, characterized by having a stratified structure, cubical packing of the chlorine atoms, crystallization in the rhombohedric system, and by deformation (as a result of the activation) such that the reflection at d=2.56 A (which is the most intense diffraction line in its X-rays powder spectrum) decreases in intensity and is replaced by a diffused halo.